Distribution and diffusion of sodium taurocholate and egg phosphatidylcholine aggregates in rat intestinal mucin

Numerical investigation of moving gel wall formation in a Y-shaped microchannel

Molecular diffusive membranes play crucial roles in the field of microfluidics for biological applications e.g., 3D cell culture and biosensors. Hydrogels provide a range of benefits such as free diffusion of small molecules, cost-effectiveness, and the ability to be produced in bulk. Among various hydrogels, Pluronic F127 can be used for cell culture purposes due to its biocompatibility and flexible characteristics regarding its environment. Aqueous solutions of Pluronic F127 shows a reversible thermo-thickening property, which can be manipulated by introduction of ions. As a result, controlled diffusion of ions into the solution of Pluronic F127 can result in a controlled gel formation. In this study, the flow of immiscible solutions of Pluronic and sodium phosphate inside a Y-shaped microchannel is simulated using the level set method, and the effects of volume flow rates and temperature on the gel formation are investigated. It is indicated that the gel wall thickness can decrease by either increasing the Pluronic volume flow rate or increasing both volume flow rates while increasing the saline volume flow rate enhances the gel wall thickness. Below a critical temperature value, no gel wall is formed, and above that, a gel wall is constructed, with a thickness that increases with temperature. This setup can be used for drug screening, where gel wall provides an environment for drug-cell interactions.

Article Highlights

Graphical abstract

Moving temporary wall in microfluidic devices

This paper describes the formation of a temporary wall between two fluid streams in a microfluidic channel. Diffusion of ions from one fluid stream into a costreaming thermally responsive polymer solution is used to lower the local gelation temperature of the polymer, leading to formation of a gel wall in the center of the flow channel. The mechanisms driving either the generation or removal of the wall on its both sides are described and discussed. This wall allows well-controlled transport of particles from one stream into the other.

Mass Transport Properties of Progesterone and Estradiol in Model Microemulsion Formulations

PURPOSE

The purpose of these studies was to determine the extent to which drug loading influences the mass transport characteristics of poorly soluble steroids from model microemulsion formulations in vitro.

METHODS

Two conditions of drug loading in the microemulsions were tested, "near saturation" and "constant loaded." Mass flux of (3)H-labelled progesterone or estradiol was measured in a side-by-side diffusion chamber from microemulsions consisting of Brij 97, Miglyol 812 and water. Pulsed gradient NMR was used to measure the diffusivities of all components. The thermodynamic activity and fraction of free drug in the formulations were measured by polymer uptake. Solute flux was calculated employing an aqueous boundary layer model.

RESULTS

Under near saturation loading, all microemulsion formulations showed significantly increased flux of steroids compared to the saturated aqueous solution. For both steroids flux values in the 0.5 and 1% systems were significantly lower for the 3% oil formulation, despite the observation that the 3% formulation held significantly more drug. On the other hand, for all the formulations under constant drug loading, solute flux was only moderately increased for progesterone and not at all for estradiol when compared to the saturated aqueous solution. Under both loading conditions, thermodynamic activities did not correlate to flux indicating some other factor was modulating mass transport. Effective diffusivities of the steroids in formulations as determined by NMR were significantly reduced compared to those of the monomer drug in aqueous solution. In both near-saturated and constant-loaded conditions, the calculated values for progesterone flux were markedly similar to those observed experimentally suggesting solubilization and diffusion events in the aqueous boundary layer had a strong influence on mass transport. In contrast, calculations for estradiol were less successful in modeling the observed flux values.

CONCLUSIONS

In systems nearly saturated with drug, the microemulsion formulation leads to a greatly enhanced rate of steady-state mass transport while in systems with drug loading far from saturation, the microemulsion formulation appears to have a minimal ability to promote mass transport. The aqueous boundary layer diffusion model was successful in fitting progesterone results but was not successful for estradiol.

Interaction of bile salts with gastrointestinal mucins

The properties of three mucins were examined to identify the structural features responsible for their functional difterences. Bovine submaxillary mucin (BSM), porcine gastric mucin (PGM), and rat intestinal mucin (RIM) were each characterized, and high carbohydrate contents were found for RIM and PGM. The amino acid compositions were typical of mucin glycoproteins, with over half comprising small, neutral amino acids. Thereafter, each mucin was equilibrated with three different series of concentrations of the bile salts sodium taurocholate, sodium taurodeoxycholate, and sodium taurochenodeoxycholate. Following multiple centrifugations, the supernatant and mucin pellet concentrations of the bile salts were measured. The bile salt pellet concentration was plotted as a function of supernatant concentration, and from the slopes, the excluded volumes were calculated as 25, 29-44, and 28 55 mL/g for BSM, RIM, and PGM, respectively. The intercepts were 8-10, 2-3, and 1-3 mM for BSM, RIM, and PGM, respectively, which represents an estimate of the bound concentration of bile salt. Differences among the bile salts were observed in the excluded volume and amount bound, but no trends were evident. The bile salts may interact as aggregates with the hydrophobic areas and carbohydrate side chains of the mucins, providing favorable sites for association. The binding at low concentrations with exclusion at high concentrations is significant for modulating the absorption of lipid aggregates from the intestine. Finally, the differences among the mucins reflect the unique structure function relationship of these gastrointestinal mucins.